Process of making articles of foamed polymethyl methacrylate

ABSTRACT

The invention relates to a process of making articles of foamed polymethyl methacrylate (PMMA). In a first process stage a mixture comprising monomeric methyl methacrylate, plasticizer, preferably liquid foaming agent and catalyst is polymerized by heating and/or irradiation to form a foaming agent-containing solid body. In the second process stage this body is foamed by heating to the softening temperature of the plastic material. Either of these stages may also be performed continuously. The final product is clear-transparent and has large closed cells, a smooth surface, is light- and weather-resistant, readily workable, has good thermal insulation properties also in combination with plastics or glass coatings and may be used in a variety of applications, especially as a transparent insulating slab or plate.

TECHNICAL FIELD

The present invention relates to a process of making articles of foamedpolymethyl methacrylate, which are in particular transparent and maytake the form of sheets, boards, shaped articles and laminated articleshaving a foamed core.

BACKGROUND OF THE INVENTION

It is known to make foamed materials from synthetic plastics such aspolyurethane. Basically, there are two processes available. In the firstprocess, the starting composition comprising monomers and foaming agentis polymerized and simultaneously foamed in a single-stage process.According to this process useful products can be obtained only if themonomers polymerize rapidly and thus form a mechanically stable productwhich may be foamed by the gases or vapours of the foaming agent,whereby stable cells are formed.

In the second known process the monomeric starting material is initiallypolymerized with the admixed foaming agent without being foamed,whereupon in a second stage the polymer is brought into a plastic orfluid condition by heating while simultaneously the foaming agent iseither evaporated or decomposed, so that gas bubbles or cells are formedin the synthetic material and a foamed plastic is obtained.

In the case of polymethacrylates none of these known processes leads tothe desired success. In the first-mentioned process, it is impossible tokeep the bubbles (cells) in a stable state during the polymerization ofthe acrylic foam. Due to the polymerization the surface tension of thebubbles increases so that they burst and combine with each other, thuscausing the foam to collapse before it is cured.

The known two-stage process also does not lead to the desired success inthe case of polymethacrylates. According to the British PatentSpecification No. 1,448,621 and the French Patent Specification No.1,423,844 a polymethacrylate is to be foamed by initially polymerizing amethacrylate with a foaming agent admixed thereto under conditions whichinhibit the activation of the foaming agent. In a second stage the curedpolymethacrylate is heated to the softening temperature at which thefoaming agent will be activated. But because of the high internalpressure the foamed plastic substantially comprises only small bubblesor cells. Experiments have shown that during foaming thepolymethacrylate composition is blown to all sides to result in a foamwith a wavy and warped surface. The products obtained therefromfurthermore have little transparency and permit the light to passtherethrough only diffusely because it is reflected by the numeroussmall bubbles or cells.

There is a demand for a clear, transparent foamed plastic on apolymethyl methacrylate basis, which has a high transparency and at thesame time good mechanical stability and high thermal insulation propertyas well as a smooth surface structure and which may be obtained in theform of sheets, boards or slabs, shaped articles and laminated ormultilayer articles having a foamed core. It is impossible with any ofthe known processes to make such a foamed plastic, which is alsofundamentally different with respect to its structure from knowntransparent extruded laminated boards of synthetic materials, whichcomprise in their interior either webs or closely adjoining very smallor capillary-type tubes.

DESCRIPTION OF THE INVENTION

It is the object of the present invention to provide a process for themanufacture of articles of foamed polymethyl methacrylate, which processresults in an especially transparent foamed plastic having closed largeplanar cells of controlled size, shape and density, and having a smooth,non-porous surface, which furthermore permits the manufacture of shapedarticles and multilayer or laminated articles.

The above-specified object is solved by the process according to theinvention, in which

(1) a composition containing:

(a) monomeric methyl methacrylate and possibly monomeric acrylate;

(b) a plasticizer;

(c) a foaming agent which neither evaporates nor decomposes at thepolymerization temperature;

(d) at least one polymerization catalyst; is prepared at a temperaturebelow the evaporation or decomposition temperature of the foaming agentand subsequently is polymerized at a pressure in excess of theevaporation pressure of the foaming agent at the polymerizationtemperature, possibly in a mould, to form a--particularly board- orslab-shaped--body, and

(2) the thus prepared foaming agent-containing solid body--possiblyafter mechanical treatment thereof--is foamed on a non-adherentsubstrate by heating to a temperature in excess of the evaporation ordecomposition temperature of the foaming agent and in excess of thesoftening temperature of the polymer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow sheet for the discontinuous process; and

FIG. 2 is the flow sheet for the continuous process.

Preferred quantities for the starting mixture are as follows:

(a) 100 parts by weight of methylmethacrylate and possibly acrylatemonomer

(b) 5 to 40 parts by weight of plasticizer

(c) 10 to 100 parts by weight of foaming agent

(d) polymerization catalyst in usual quantity.

What is of especial importance to the process of the present inventionis the use of the plasticizer (b), which is preferably a monomeric alkylmethacrylate with the alkyl group including at least 3 C-atoms,preferably an n-butyl methacrylate monomer or an ethylhexylmethacrylatemonomer, wherein this plasticizer is used in a quantity of preferably 5to 25 parts by weight, and most preferably 10 to 15 parts by weight per100 parts by weight of the methyl methacrylate monomer (a). By means ofthe quantity and type of the added plasticizer it is possible to controlthe softening properties of the foaming agent-containing solid body ofpolymethyl methacrylate obtained on polymerization and thus to controlthe foaming temperature as well as the size and number of cells and themechanical properties of the obtained article of foamed polymethylmethacrylate, particularly the strength and elasticity thereof.

The foaming agent used is preferably a conventional liquid inert foamingagent having a boiling point below 100° C., particularly a saturatedhydrocarbon or fluorocarbon, preferably butane, pentane, hexane or,respectively, trichlorofluoromethane or trichlorotrifluoroethane,because the boiling points of such foaming agents are above ambienttemperature so that blending of the starting mixture may simply beperformed in an agitated vessel at atmospheric pressure. The use of asolid foaming agent which supplies expansion gases upon decompositionthereof is less suitable, because such foaming agents do not result inclear, transparent foamed plastics and since they particularly permitthe formation of only small cells which cause the foamed plastic toappear opaque. Moreover, solid foaming agents normally have adecomposition temperature in excess of 100° C. and therefore requirehigher foaming temperatures.

It is within the scope of the process according to the present inventionto perform the polymerization in any manner known per se for methylmethacrylate with the respective catalysts.

As the polymerization catalyst (d) there may especially be used aconventional peroxide catalyst which becomes active on being heated. Ifit is required for the polymerization to effect heating at a temperaturein excess of the evaporation temperature of the foaming agent, thepolymerization will naturally be effected under pressure in order toprevent premature liberation of the foaming agent. Although the use ofsolid foaming agents together with such a peroxide polymerizationcatalyst is possible, it is not preferred.

According to a particularly advantageous embodiment of the process ofthe present invention a photo-initiator is used as the polymerizationcatalyst, which permits polymerization of the composition at lowtemperature, especially approximately at room temperature, by means ofexposure to a radiation matched to the photo-initiator, e.g. ultravioletlight of 350 nm in the case of a photo-initiator known for acrylatemonomers. In this embodiment of the polymerization it is possibleespecially to control the polymerization reaction by energization andde-energization of the sources of radiation in such a way that thetemperature in the polymerizing composition does not exceed apredetermined value during the exothermic polymerization reaction, saidpredetermined value being just below the evaporation temperature of theliquid foaming agent. Control may be effected in a simple way by using athermocouple. Of course, in case of photo-polymerization the foamingagent-containing composition to be polymerized should be accessible tothe radiation, i.e, it should either present a free surface or should beirradiated while disposed on a radiation-transmissive surface on one orpreferably either side thereof while sandwiched between two suchsurfaces. As radiation-transmissive substrate or cover, preferably glassplates or sheets, are employed between which a space accommodating theliquid blend is defined by means of a sealing strip. Photopolymerizationmay be effected with layer thicknesses of from approx. 1 to more than 50mm; preferably the layer thickness is within the range of from 1 to 10mm, resulting after foaming in articles having a layer thickness of from4 to 50 mm.

When performing polymerization in the specified manner with liquidfoaming agents, which may also take place in a mould, an intermediateproduct in the form of a glass-clear transparent strong solid body offoaming agent-containing polymethyl methacrylate is obtained, thedensity of which depends on the type and quantity of the foaming agentused. Trichlorofluoromethane, for instance, results in a polymer havinga typical density of 1.45 g/cm³. Just like conventional polymethylmethacrylate, this intermediate product may be mechanically treated, inparticular it may be cut and sawn. It leads itself conveniently totransportation and storage if it is not immediately delivered to thesecond process stage, in which it is foamed by heating to a temperaturein excess of the evaporation or decomposition temperature of the foamingagent and in excess of the softening temperature of the polymer.

In this second process stage the foaming agent-containing solid body isretained on a non-adherent substrate so that its two-dimensionalexpansion both in longitudinal direction and widthwise thereof uponfoaming is not impeded and the formation of large closed cells in thefoamed material is permitted. A PTFE-coated glass or metal surface isespecially suitable as such a non-adherent substrate.

If the expansion of the foamed object in upward direction is to belimited, a cover surface, which suitably also has an anti-adhesivecoating thereon, is provided at an appropriate distance from thesubstrate, the distance between both surfaces appropriately being notsubstantially less than the height of the foaming agent-containing solidbody when foamed unrestrictedly, so that the formation of large cells isnot obstructed. When the foamed body contacts the cover surface thecells both at this surface and at the surface of the substrate will beflattened so that, if desired, a substantially planar surface of thefinished article of foamed polymethyl methacrylate will be obtained.

It is particularly pointed out that both the polymerization of thecomposition in the first process stage and the foaming in the secondprocess stage may be performed either separately or immediatelysucceeding one another continuously on a non-adherent conveyor belt orbetween two such smooth conveyor belts, which are moved in the samedirection at an approximately constant spacing relative to each otherand parallel to one another.

The discontinuous process illustrated in FIG. 1 is carried out as setforth in Example 1. The feed containing the components a, b, c and d aremixed in a mixing vessel and then the mixture is positioned betweenglass plates in the polymerization apparatus and polymerized. The glassplates are then separated and the polymerized slab removed and cut intoplates of the desired shape in the mechanical working station. A cutpolymerized plate is then placed between larger glass plates and foamedby heating in the foaming station to produce the finished foamedproduct.

In the continuous process illustrated in FIG. 2, the feed components a,b, c and d are mixed and positioned in the polymerizing station betweentwo non-adherent smooth conveyor belts and polymerized in thepolymerizing station between the two conveyor belts which are moved inthe same direction at an approximately constant spacing relative to eachother and parallel to each other into the foaming station wherein thepolymerized continuous sheet is foamed and continuously passed out ofthe foaming station in the form of finished product.

Size and shape of the cells are influenced, on the one hand, by the kindof the starting composition, particularly the type and quantity of theingredients a, b and c thereof, by the softening temperature of thepolymer of the first stage and the expandibility thereof at the foamingtemperature and, on the other hand, by the rate of heating in the secondprocess stage during foaming as well as by the temperature used forfoaming. Generally, when a liquid foaming agent having a boiling pointof less than 100° C. is used, a foaming temperature of about 110° C.,which may possibly also be higher such as about 160° C., and a foamingtime of about 30 minutes will be sufficient.

The higher the temperature during foaming the more rapidly foaming willtake place and the larger the cells or bubbles obtained will be. At alower temperature the foaming will proceed more slowly, but large cellswill also be obtained provided said lower temperature is maintainedsomewhat longer. Generally, the size of the cells will increase thelonger a predetermined constant temperature is maintained. Aparticularly uniform foaming will be obtained when the temperaturegradient in the composition is kept as small as possible, e.g. when thecomposition is heated by means of high frequency. For the rest, athicker board or slab will have to be heated longer to obtain completefoaming thereof than a thinner board or slab of the same composition.

The density of the finished foamed article obtained after cooling may beset at 0.03 to 0.05 g/cm³.

The articles obtained in accordance with the invention from foamedpolymethyl methacrylate are characterized by relatively large closedcells having a diameter of about 5 mm and defined by thin walls so thatthe clear, transparent character of the polymethyl methacrylate is alsodistinctly retained in the foamed material and the latter exhibits veryhigh transparency and appears clear, which is in contrast to the opaqueappearance of foamed materials having small cell size and multiplereflection of light caused thereby. The mechanical and physicalproperties of the thus obtained foamed material of polymethylmethacrylate are excellent--in accordance with those of the basicmaterial polymethyl methacrylate--for the most varied applications. Theproperties to be particularly stressed are its good strength andelasticity, good workability, rot-resistance, light- andweather-resistance and a substantially smooth surface on those sideswhere the foam contacted a non-adherent surface or substrate duringformation thereof. Such articles are therefore especially suitable forconstructional applications, as thermal and low-temperature insulatingslabs and domelights, and also for ornamental purposes. Moreover,already during foaming of the foaming agent-containing solid body orafter completion of such foaming operation the resulting finished foamedarticles may be provided, coated or bonded with a backing of transparentsheets or boards made of plastic and glass, respectively, so thatself-supporting laminated bodies having high transparency and high heatinsulation property are obtained thereby, which may advantageously beused for a variety of constructional purposes, in particular for greenhouses.

If desired, colourants and/or filler materials or pigments may beadmixed to the starting composition for preparing coloured, clearlytransparent, opaque or non-transparent articles which exhibit the abovespecified advantageous properties.

EXAMPLE 1

600 g of monomeric methyl methacrylate (MMA), 72 g of n-butylmethacrylate monomer (BMA), 180 g of trichlorofluoromethane (b.p. 24°C.) as the foaming agent, 1.2 g of benzoyl peroxide as the heatpolymerization catalyst, and 1.2 g of a conventional photo-initiator areblended in an agitated vessel at a temperature of less than 20° C.

The mixture is poured onto a planar glass plate provided with ananti-adhesive coating into the area defined thereon by a sealing tapehaving a height of 4 mm, so that the layer of the liquid mixture reachesthe upper edge of the sealing tape. Then a second planar glass plate isplaced thereon, which is also provided with an anti-adhesive coating onthe side facing the composition.

Both glass plates with the mixture provided therebetween are heldtogether by means of removable clamps, and the assembly is introducedinto an exposure chamber where it is polymerized by means of ultravioletfluorescent lamps having a spectrum which is optimum for the selectedphoto-initiator, e.g. fluorescent lamps Philips TW/05 which have aspectrum of substantially 350 nm, while the temperature of thecomposition is maintained just below the boiling temperature of thefoaming agent, i.e., 24° C., by interrupting the exothermicpolymerization reaction, when this desired temperature is reached, byturning off the fluorescent lamps for a predetermined period of time oruntil the temperature falls below 22° C. The polymerization carried outin this way is completed after about one hour. The glass plates areseparated from each other, and a glass-clear, strong, solid slab offoaming agent-containing methyl methacrylate polymer having a density of1.21 g/cm³ is removed.

The obtained slab is foamed by heating either as produced or aftercutting into plates of desired size. Such a glass-clear, solid, foamingagent-containing polymer plate of 117×117×4 mm is placed between twoconsiderably larger glass plates which are coated on their facing sideswith PTFE as an anti-adhesive agent and are maintained at a spacing of18 mm. This assembly is completely foamed in a furnace at 110° C. for c.30 minutes, wherein the polymer plate will expand two-dimensionallyparallel to the glass plates and also with respect to height until thesurface of the foamed plate abuts the upper glass plate. Aftercompletion of the foaming step the still warm plate of foamed polymethylmethacrylate removed from between the glass plate assembly is eitherpermitted to cool on a planar surface, or it is removed from the glassplate assembly after cooling thereof as a whole. The thus obtained plateof foamed polymethyl methacrylate has a size of 330×330×18 mm and adensity of 0.028 g/cm³. It is rigid, slightly elastic andclear-transparent with closed pores having a diameter of about 5 mm.This plate is highly suitable as a good light-transmissive insulatingmaterial which may be placed between transparent plates made of glass orplastic and may possibly be bonded thereto so that self-supportingtransparent insulating slabs, particularly for constructional purposes,domelights and the like are obtained.

EXAMPLE 2

In the starting mixture according to example 1 the photoinitiator wasreplaced by the same quantity of heat polymerization catalyst (benzoylperoxide), and polymerization was performed within a pressure chamberwhile heating to 85° C. at a pressure in excess of the evaporationpressure of the foaming agent at this temperature. The polymerizationwas completed after about 60 minutes. After cooling a glass-clear strongsolid plate of foaming agent-containing methyl methacrylate polymer wasremoved from the pressure chamber, which plate was foamed in the sameway as described in Example 1 and resulted in a similar product.

EXAMPLE 3

In the mixture according to Example 1 the 72 g of n-butyl methacrylatewere substituted by 72 g of n-butyl acrylate. The mixture waspolymerized in the same manner as in Example 1, and the obtained solid,foaming agent-containing plate of methyl methacrylate polymer was foamedas in Example 1 and resulted in a foamed polymethyl methacrylate havingthe same properties as the product of Example 1.

EXAMPLE 4

In the mixture of Example 1 the 180 g of trichlorofluoromethane weresubstituted by 180 g of trichlorotrifluoroethylene (b.p. 45° C.).Polymerization was effected in the same way, but at a temperaturebetween 40° and 42° C. within 40 minutes. The obtained glass-clear,solid, foaming agent-containing plate of methyl methacrylate polymerresulted upon foaming according to the procedure of Example 1 in aproduct similar to that of Example 1.

EXAMPLE 5

In the mixture of Example 1 the 180 g of trichlorofluoromethane weresubstituted by 180 g of n-pentane (b.p. 36° C.) as the foaming agent.The mixture was polymerized as in Example 1, and the obtained solid,foaming agent-containing plate of methyl methacrylate polymer was foamedas in Example 1 at 110° C. A hazy foamed polymethyl methacrylate havinga density of about 0.032 g/cm³ was obtained.

EXAMPLE 6

0.6 g of trimethylolpropane trimethacrylate was added to the mixture ofExample 1 as a cross-linking agent (0.1% based on monomeric methylmethacrylate). The mixture was polymerized in the same way as in Example1, and the obtained solid, foaming agent-containing plate of methylmethacrylate polymer was foamed as in Example 1, but at a temperature of100° C. While at this concentration of the cross-linking agent it isstill possible to foam the solid, foaming agent-containing plate, oneobtains a foamed product having small bubbles or cells and a density ofabout 0.035 g/cm³.

I claim:
 1. A process of making articles of foamed polymethylmethacrylate comprising(1) mixing(a) 100 parts by weight of monomericmethyl methacrylate; (b) 5 to 40 parts by weight of a plasticizer whichis a monomeric alkyl methacrylate in which the alkyl group comprises atleast 3 carbon atoms; (c) 10 to 100 parts by weight of a foaming agentthat will neither evaporate nor decompose at the polymerizationtemperature and which is a liquid inert foaming agent boiling at atemperature of less than 100° C.; and (d) at least one polymerizationcatalyst; at a temperature below the evaporation or decompositiontemperature of the foaming agent to form a mixture and polymerizing saidmixture at a pressure in excess of the evaporation pressure of thefoaming agent at the polymerization temperature to form a solid bodycontaining said foaming agent, and (2) positioning said foamingagent-containing solid body on a non-adherent substrate and heating saidsolid body to a temperature in excess of the evaporation temperature ofthe foaming agent and in excess of the softening temperature of thepolymer to foam said solid body and form a transparent foamed polymethylmethacrylate with closed cells having a diameter of about 5 mm. and adensity of about 0.03 to 0.05 g/cm³.
 2. The process of claim 1, whereinsaid monomeric alkyl methacrylate plasticizer is selected from the groupconsisting of butyl methacrylate, ethylhexylmethacrylate monomer andn-butyl acrylate and is in an amount of from 5 to 25 parts by weight ofsaid mixture; and wherein said foaming agent is a fluorocarbon orhydrocarbon foaming agent.
 3. The process of claim 2, wherein saidfoaming agent is selected from the group consisting oftrichlorofluoromethane, trichlorofluoroethane, butane, pentane andhexane.
 4. The process of claim 1, wherein said polymerization catalystis a catalyst effecting polymerization of the mixture upon heatingthereof, and said polymerization is carried out by heating said mixtureand maintaining said mixture at a pressure in excess of the evaporationpressure of the foaming agent at the polymerization temperature.
 5. Theprocess of claim 4, wherein said polymerization catalyst is a peroxidecatalyst.
 6. The process of claim 1, wherein said polymerizationcatalyst is a photo-initiator causing photopolymerization of themixture, and said polymerization is carried out by irradiating saidmixture with radiation which causes polymerization and wherein thepolymerization temperature is maintained below the evaporationtemperature of the foaming agent at the pressure during polymerization.7. The process of claim 6, wherein said radiation is ultraviolet light.8. The process of claim 1, wherein said mixture is polymerized in amould at a pressure in excess of the evaporation pressure of the foamingagent at the polymerization temperature to form said solid body.
 9. Theprocess of claim 1, wherein said foaming agent-containing solid bodyprepared in step (1) is subjected to mechanical working before beingfoamed on said non-adherent substrate by heating to a temperature inexcess of the evaporation temperature of the foaming agent and in excessof the softening temperature of the polymer.
 10. The process of claim 1,characterized in that said polymerization is conducted within a spacewhich is defined by a sealing type between two smooth surfaces of twoplates extending at equal spacing relative to one another, said sealingtape forming a closed curve and continuously contacting either of saidplates along said curve, whereby a slab or plate shaped foamingagent-containing solid body is obtained.
 11. The process of claim 10,wherein said two plates are glass plates.
 12. The process of claim 1,wherein said polymerization of the mixture is conducted continuouslywithin the laterally confined space between two smooth non-adherentsurfaces of conveyor belts which extend parallel to and at approximatelyequal spacing from one another and move in the same direction.
 13. Theprocess of claim 1, wherein the foaming of the foaming agent-containingsolid body obtained in the first process step is carried out by heatingsaid solid body on a non-adherent substrate at atmospheric pressure, inthe second process step.
 14. The process of claim 1, wherein the foamingof the foaming agent-containing solid body is conducted within a mouldcomprising non-adherent mould walls, the spacing between said wallsbeing slightly smaller than the maximum layer height of the article offoamed polymethyl methacrylate obtainable by unrestricted foaming. 15.The process of claim 12, wherein the foaming of the continuouslyobtained slab or plate-like foaming agent-containing solid body isperformed continuously by heating said solid body on a non-adherentsubstrate to a temperature in excess of the evaporation temperature ofthe foaming agent and in excess of the softening temperature of thepolymer.
 16. The process of claim 1, wherein said 100 parts of thecomponent (a) includes monomeric acrylate.